One of our many service kits is this Thermo housing bolt kit with gasket.  In most cases, the original housing bolts are hour glass shaped and corroded bad enough where they can't be used again without risk of breaking off. Our grade 8 Gold cadium plated bolts and washers will step right up to the plate along with a nice new gasket. 
B&B TECH TIP:  Always inspect the lower plate for rusting out or corrosion when changing out the Thermostat housing.